Effect of Finish Rolling Temperature on Microstructure and Mechanical Properties of Low-Si Steel After Quenching and Nonisothermal Partitioning

The quenching and nonisothermal partitioning process has been found to achieve excellent combinations of strength and ductility through the retention of austenite in the microstructure. Therefore, in the present study, the effect of grain size on phase transformations and ensuing mechanical properties has been investigated. A low-Si and Al-free steel was finish rolled at different temperatures which was subsequently quenched to a constant temperature below Ms for the formation of martensite with remaining untransformed austenite. This was followed by slow cooling to room temperature to simulate the condition of nonisothermal partitioning of carbon from martensite to the austenite. The results show that there is not much change in the average grain size with variation in the finish rolling temperature, as well as, no measurable differences are observed in the microstructural features and retained austenite content. The microstructure shows the presence of martensite, carbides, and a small amount of retained austenite. However, in terms of mechanical properties, the finish rolling at a lower temperature of 900 °C lead to higher hardness and strength with comparable ductility in comparison to the sample which has been finish rolled at a relatively higher temperature (940 °C). The above results will thus be helpful in designing the industrial processing schedule for low-Si steel to achieve enhanced performance.